Techniques for heating, and subsequently drawing, glass into fine fibers has been known for millennia. It was, however, during the 1930s when this technique was first introduced into the textile industry. As further explained below, this technique was employed later, during the 19th century, to manufacture glass optical fibers.
Light guidance in transparent pipes and water streams historically inspired the use of optical fibers for light transmission. The light guiding process using the total internal reflection was first demonstrated by Daniel Colladon and Jacques Babinet in Paris during the early 1840s. It found applications such as illumination in dentistry, image transmission and internal medical examination early in the 20th century. Later, during the 1920s, the concept of modern glass fibers with a glass core and a lower index cladding for a more suitable index guiding was introduced. Low-index oils and waxes were mostly used to produce the lower-index cladding. During the 1950s, University of Michigan undergraduate student Lawrence E. Curtis produced the first glass-core fiber having glass cladding for minimizing the interference of guided light with the surrounding environment. Advances in the fiber fabrication process combined with the proper choice of glass materials rendered the optical fibers as feasible tools for long-distance optical telecommunications, as well as many other applications such as sensing and imaging. During the 1990s, micro-structured fibers and photonic crystal fibers were developed wherein the guiding mechanism was based upon light diffraction from periodic structures in the fiber. It was found that photonic crystal fibers could potentially transmit higher light power and would give the possibility of dispersion adjustment based on structure design. In recent years, a new class of fibers (so-called multi-material fibers) emerged based on thermal co-drawing of multiple types of materials all having thermally and mechanically compatible properties. This new class of fibers enabled the introduction of novel functionalities (i.e., not limited to optical light transmission). For example, one novel expanded functionality included fibers with semiconducting glass and metal electrodes integrated into a single fiber for light detection applications. The field of multi-material fibers recently expanded even further to include piezoelectric fibers and multi-material fibers for structured microsphere and nanosphere fabrication.
Throughout the history of fiber development, thermal fiber drawing has been the most popular and the most successful fabrication method. Simplicity and speed of thermal fiber drawing made optical telecommunication an economically viable technology. The circularly symmetric geometry of optical fiber fabrication was indeed inspired by the natural shape of water streams and glass fibers that were produced through heating and pulling of glass.
In the fiber drawing process, softened material has the tendency to round up into fibers having a circular cross-section to minimize the surface free energy under surface tension. However, in the longitudinal direction, the tension along the fiber that is produced by the intentional pulling process, dominates the surface tension and leaves the fiber longitudinally elongated. During the pulling process, material is maintained at or about the softening temperature for a (brief) period of time adequate to stretch it into a fiber. It is then gradually cooled to solidify the stretched form that is called a fiber. This is the fiber fabrication process that has been used for centuries in the textile industry and for decades in the field of optics. In recent years, fibers having non-circular cross-sectional areas have been created by giving an asymmetric geometry to the fiber preform and trying to maintain that geometry by not overly heating the fiber during the drawing process. It is possible to maintain non-circular structures by not giving the material enough freedom (low viscosity) and time to round up to a circular shape. Fibers made using this method—having hexagonal, square, rectangular and even D-shaped cross-sections—have been reported for various applications. With regard to all fibers of different materials for various applications over decades, the circular symmetry of fiber preform heating has allowed for equal scale reduction in both transverse directions (i.e., height and width) across the fiber. This results in maintenance of the aspect ratio of the preform in the final drawn fiber by allowing equal shrinkage in both transverse directions
In conventional fiber drawing methods, and as illustrated in FIG. 1, a fiber preform 110 is placed in a tubular (or similar) furnace 140 wherein a heating element 142 surrounds the preform 110 in a manner that provides uniform heating about the preform 110. The furnace begins heating and softening the materials from the outermost layers to the innermost layers of the preform 110. As a result, there is always a relatively large temperature (and hence viscosity) gradient across the fiber preform 110 in the radial direction ‘B’ (transverse to the longitudinal axis ‘A’). The middle of the preform 110 (coinciding with the center of the furnace 140) has the highest viscosity, while the outer layers have relatively lower viscosities. This viscosity gradient acts to force the material to flow toward, and stretch from, the center of the preform 110, where the temperature is lowest and the viscosity is highest. This can be confirmed by dislocating the preform 110 in such a manner that its center no longer coincides with the center of symmetry (or more accurately the coldest point) of the furnace 140. In this case, the material flow and stretching occurs from the location of the lowest furnace temperature that is not at the center of the preform 110. This is commonly referred to as “asymmetric fiber pulling,” which is not desired for most fibers. To minimize this problem most fiber draw machines have automatic centering features that align the preform 110 with the center of furnace 140.
If the preform does not have a cylindrical shape, it will still shrink almost uniformly in both transverse directions and will maintain the original shape in a smaller scale, unless temperature is too high and therefore viscosity is too low. In that case, the material has a tendency to minimize its free energy under surface tension. This will deform the fiber's cross-section towards a circular one.
However, the use of a furnace that supplies a uniform thermal gradient about 360 degrees of a preform limits the shapes that can be drawn. Therefore, a method of applying thermal gradients to facilitate cross-sectional shapes not possible with circular heating is needed.